In Biggles in the Blue Johnston was a chemist who had a shop on Aclim Street, Kingston. Commander Evans had recommended Johnston to Werner Wolff when he had asked about a good chemist to develop photographs.
During their hunt for Wolff's secret papers, Ginger and Bertie decide to call on Johnston, reasoning that Johnston might be able to describe the scenery in Wolff's photos and so give a clue as to where he had been. Accordingly, Ginger asked Johnston if he had printed any photos for Wolff recently involving flamingos, since Wolff had promised to take some shots of those birds for Evans. Johnston confirmed that he did some work for Hagen (Wolff's assumed name) but it had been some time ago and there had never been any photos of flamingos.
However, as Ginger and Bertie turned to leave, Johnston told them that one other customer had sent in a spool of film with photos of flamingos just one or two days ago. While Johnston did not keep copies of the prints, he was able to give Ginger and Bertie the identity of this customer--Mr Stalling, von Stalhein's cover name! He had beaten Biggles to Wolff's camera and developed the film which would yield vital clues about where he had been and where he had buried his secret papers.